Electrolytic soft gold plating

ABSTRACT

Characteristics of soft gold plating produced by electrolysis are improved by adding trace amounts of lead to the gold plating solution and by replenishing such content during use. Such characteristics include improved brightness and uniformity of deposits, better defined crystallinity of deposits, enlarged current density range, and extended solution life. Lead content generally less than about 1 ppm, based on the solution, does not affect purity of gold deposit as measured by ordinary analytical techniques.

United States Patent [1 1 Reinheimer 111 7 3,833,487 [45] Sept. 3, 1974 ELECTROLYTIC SOFT GOLD PLATING [75] Inventor: Horst Alfred Reinheimer, Reading,

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

OTHER PUBLICATIONS P. F. Thompson, J. Electro Chem. Soc., Vol. 91, pp.

Primary Examiner-G. L. Kaplan Attorney, Agent, or Firm-G. S. Indig [5 7] ABSTRACT Characteristics of soft gold plating produced by electrolysis are improved by adding trace amounts of lead to the gold plating solution and by replenishing such content during use. Such characteristics include improved brightness and uniformity of deposits, better defined crystallinity of deposits, enlarged current density range, and extended solution life. 'Lead content generally less than about 1 ppm, based on the solution, does not affect purity of gold deposit as measured by ordinary analytical techniques.

11 Claims, 1 Drawing Figure -sooP ElmnA/cnn L E A D(-MgD|F|E0 Pb pp J/ Z LLI s v Q.

5 NEW BATH O E -9oo 2 3 L USED BATH /.--1--''T .-|ooo/ lOmA/cm -uoo l I I o 5 l0 l5 TIME MINUTES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is concerned with soft gold electrolytic plating, i.e., plating of high purity gold.

2. Description of the Prior Art Electrolytic soft gold plating is of appreciable significance in the fabrication of piece parts and circuit apparatus, e. g., semiconductor devices, integrated circuitry, etc. Purposes served by such deposited pure gold layers are protection of devices, conducting electric current and bonding.

Due to increasing use and to more and more discriminating needs accompanying device miniaturization, the ancient art of electrolytic gold plating has been undergoing reexamination by many worker s. As a result, commercial bath compositions are now available for operation over a variety of pH values, and applicability of such solutions has been extended. Good quality electronic grade gold solutions as made now typically contain as little as of the order of parts per million of unintentional inclusions. A well engineered variety of buffer salts, including citrates, phosphates, sulfates, carbonates, acetates, etc., and sometimes salts designed to improve ionic conductivity as well, permit plating under conditions best utilized for the particular class of apparatus being fabricated.

Based in part on economic considerations, gold plating solutions ordinarily include gold in the form of a soluble complex-generally potassium or sodium cyanoaurate (potassium or sodium gold dicyanide). Use of such a complex permits reasonable plating rates from solutions containing relatively small amounts of gold. Conventional procedure calls for replenishing such bath during use by additions of the same salt. It is common toreplenish after a small fraction of the gold content has been depleted and to carry out such replenishment many times so as to have equivalent turn arounds of at least 10 or many tens.

Electrolytic soft gold deposits produced from commercial solutions under well controlled conditions are generally of sufficient purity. Morphological character istics are, unfortunately, variable, however, and best platings (bright yellow and of uniform fine crystallinity, I

etc.) have been obtainable only from relatively fresh solutions and, even then, generally only when operating at less than fairly low current densities.

General experience indicates the obtainability of adequate deposits only up to one or two turn arounds. Attempts to regenerate plating solutions have met with inconsistent results. Regeneration has often been based on assumed contamination, and regeneration procedures have included, e.g., use of activated carbon and filtration.

SUMMARY OF THE INVENTION replenishment at frequent intervals, not exceeding the equivalent of three turn arounds in gold content. More frequent and smaller replenishments, however, may be preferable. Platings show no significant variation in appearance or other characteristics during long bath use and, particularly over the low range of lead addition (up to about 500 ppb), show no detectable lead content as measured by ordinary analytical techniques.

Maintenance of lead content within the prescribed limits permits regular attainment of current densities of 20 milliamperes per cm in apparatus provided with conventional stirring means. Increased agitation permits higher densities without change in deposition morphology.

Gold plating baths to which the invention is applicable include those having a pH range of from 4.0 to 14.0.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE, on coordinates of cathode potential in millivolts and time in minutes, shows the relationship between these two parameters fora plating solution maintained in accordance with the invention as well as for two unmodified solutions, one freshly prepared and the other after extensive use. Two plots, one for a density of 3 mA/cm and one for 10 mA/cm are included.

. DETAILED DESCRIPTION 1. The FIGURE and Pos'tulated Mechanism The curves depicted are characteristic of a very large set of data measured under a variety of plating conditions and using compositions representative of commercial use. The particular composition utilized for the data represented on the FIGURE was made-up from 20 grams per liter of potassium dicyanoaurate (KAu(CN) 100 grams per liter of monobasicpotassium phosphate (KH PO with pH adjusted to 10.0 by use of potassium hydroxide. Plating was carried out in a one liter solution maintained at a temperature of 60 to C on a 10 cm cathode ata constant current equivalent to a density of either 3 or 10 milliampereslcm Ordinate values represent cathode potential in millivolts as measured using a Luggin capillary. Abscissa values are time in minutes. Curve 1, corresponding with a fresh solution of the described composition, resulted in a bright yellow uniform coating at a current density. of 3mA/cm The cathode potential increased from a nucleation value of about 900 millivolts to an ultimate value of approximately -700 millivolts, with this final stabilized value being obtained only after a period of approximately 25 minutes. Curve 2 is'plotted from an old solution having been replenished with gold dicyanoaurate to about three turn arounds. Plating under the conditions noted was coarse and dark and generally unacceptable for electronic device use. It is seen that the plotted data commences at a nucleation cathode potential of a lesser value than the 900 millivolts of Curve 1. Continuing plating over the period represented resulted in no significant change in cathode potential. In fact, the particular experiment from which the data was taken wascarried on for a period of the order of 1 hour with no further change in cathode potential. Curve 3 is plotted from data taken from a plating procedure as described but utilizing a solution which has been modified by the addition of 500 parts per billion of lead added as lead (II) oxide in 0.1n KOH solution. It is seen that nucleation again occurred at a cathode potential value of the order of --900 millivolts.

Under the conditions noted it, however, increased rapidly to a stabilized yalue of about 680 millivolts (this value was attained in about 3 minutes) with no further change in cathode potential resulting during the time period noted. This procedure, too, was continued over a prolonged period with no significant further change in cathode potential. In fact, such a bath regularly replenished with lead to a level of 500 parts per billion at intervals corresponding to fractional gold tum-overs of about percent for a period corresponding with greater than 50 tum-overs in gold produced no further change in cathodepotential.

The FIGURE is believed to give an indication of the governing mechanism resulting from lead maintenance in accordance with the invention. As indicated, curves of the type shown were consistently maintained under all conditions and using all types of compositions. Bright deposits characterized by lateral growth of high crystallinity were observed where cathode potential was 900 millivolts and more positive, while dark brown coatings indicating outward random growth were obtained at values more negative than 900 millivolts. This data suggests that the trace lead content serves as a depolarizing agent during-deposition.

The data did indicate that the lead ions function as a very effective depolarizer since, as shown in the data plotted, 500 parts per billion reduced the cathode polarization by about 240 millivolts at pH 10 as compared with the old solution (which was apparently lead depleted). By contrast, it may be calculated that an inv (This may be calculated from the Nerst equation, see

for example, Atti, Della Academia Delle Scienze di Torino, Classe di Scienze Fisiche, Matematiche e Naturali, Vol. 99, p. 1111 (1965).)

Another significant characteristic difference evidenced by Curve 3 is the rapid potential stabilization at constant current level. This curve is characteristic in indicating stabilization in a period of minutes after the onset of current flow to a constant, less negative cathode potential (assuming constant pH, temperature, current density, agitation). Commercial soft gold plating baths which have not been modified in accordance with the invention may take as much as 40 minutes to reach a stabilized cathode potential under similar plating conditions. g

2. Compositional Considerations A. Conventional Compositions A conventional soft gold plating bath intended for electrolytic plating contains only a gold complex salt and salts as required for buffering to a desired .pH and for attaining required ionic conductivity. Conventional baths are aqueous. Unintentional contaminants, e.g., silver, iron, nickel, and cobalt, are ordinarily very low. A level of 0.01 percent based on emission spectroscopic and atomic absorption analyses is typical. In common with general practice, bath composition is in terms of g/l' (grams of solid per liter of plating solution).

Gold approximately 3 g/l to solubility limit. The most common salt for plating electronic devices is potassium dicyanoaurate, KAu( CN) A common alternative, the corresponding sodium complex salt, is generally undesirable for electronic purposes since it may result insodium contamination. The low limit of about 3 g/l permits a plating current-density of approximately 2 mA/cm. In general, lower gold content is uncommon, due to the limited current density range and frequent need for replenishment. While the'absolute limit isthe solubility limit corresponding with about 145 g/l for KAu(CN) at room temperature, a lower preferred maximum is usually specified. This preferred maximum is about 120 g/l and is dictated by the desire to minimize loss of gold through dragout (significant loss of gold in solution in the wetting layer on the withdrawn cathode). A gold salt content of 20 g/l is sufficient for plating to a practical maximum rate under most conditions and was used for the solutions reported in examples herein. This maximum rate corresponds with a current density of about 9 mA/cm for unmodified commercial baths (beyond which platings generally have undesirable characteristics) and with about 40 mA/cm for a modified bath in accordance with the invention,

with this latter rate being determined by practically attainable agitation.

Additional salts 25 g/l 250 g/l. These additional salts are incorporated for either of two reasons; to attain (and maintain) desired pH, and/or to maintain desired ionic conductivity level. Where a buffer salt system is incorporated, it may inherently increase the conductivity to the desired level thereby eliminating need for a conductivity-increasing constituent. It will be recognized that the limits indicated are primarily practical. A minimum of 15 g/l of usual salts such as phosphate, citrate, or acetate assures a conductivity of the order of 0.015 -0.025 Mhos at a temperature of 25C. This minimum is also generally required. for most buffered systems to produce sufficient buffer action to maintain pH over reasonable life at reasonable plating rates. The indicated maximum exceeds the quantity of buffer ordinarily required to maintain pH during expected life. As an example, the solutions used for the plating procedures which resulted in the data plotted for the FIG- URE were buffered to a value of pH 10.0 by use of 100 g/l of Kid- P0, (corresponding with 70 g/l of P0 together with about 50 g/l of KOH. There is a large number of buffer salts in use, and sufficient experimentation has been conducted to conclude that no limitation is imposed on the class by the invention. Exemplary salts include the dibasic and tribasic phosphates (generally potassium, ammonium sodium is avoided for the same reason that it is undesirable asthe cation in the gold complex) as well as ammonium salts including citrate, sulfate, phosphate, potassium carbonate, potassium bicarbonate, potassium acetate, potassium cyanide, and corresponding acids such as phosphoric, citricand acetic acid, etc. The basic member of the buffer system, where ever needed, is commonly potassium, hydroxide, although other alkaline material may be utilized. Tests have also been conducted successfully in unbuffered potassium dicyanoaurate solution at higher pH (10 tol3). Salts which may be used for increasing conductivity without having a significant effeet on pH include potassium sulfate, potassium cyamate, and potassium formate.

Commercial gold electrolytic baths for electronic purposes have been analyzed as containing of the order of parts per million (generally less than 1 ppm) total impurity content when freshly made up. Based on the fact that fresh baths produce bright deposits only at After prolonged use, a bath may attain an impurity content sufficiently high to harden the resultant deposit. Offensive impurity, e.g., Ni, is conventionally removed by reducing pH to a level of about 6.5 and then buffering to the desired operating level.

out agitation, while good deposits may be produced with current densities of up to about 25 mA/cm with conventional agitation.

The lead modification, in accordance with the inven- 13, L d Additi 5 tion, has no significant effect on temperature. In com- AS indicated, lead level in terms f parts f metal per mon with unmodified gold solutions it is desirable to liter of solution is, from the standpoint of maintenance, operate wlthm the range of from to 9 3 desirably maintained at a minimum of 100 ppb, ale range defined as from 60 to 85 Ab0ve though 20 ppb results in improvement. This amount 85 C, loss ofso1ut1on volume through evaporation behas been found to result in a marked improvement on 10 comes a flgmficant concern whlle,deps1ts produced platings produced from fresh solution and it is also a Pelow 60 are hkely to Show harqenmg due to a change sufficient level at which to maintain solutions during m giowflizmorphology and poslslbly alsfo to carboln use. A maximum of about 2 ppm on the same basis is c.luslon' 00m .temperature p or examp prescribed since appreciably larger amounts tend to likely to result m Knoop hardness of the order 170 produce platings of inferior morphology. This is a nonfrom a bath Whlch produces. a depqslt at preferred limit, however, for many purposes since plat- 65 40C wen fl the i prescnbed maxmum ings may contain detectable levels of lead. Hardness of of 90 Knoop, for i gold platings produced from solution containing 2 ppm lead Lead modlficatlon lmposes no ,resmctlons on other is about 90 on the Knoop scale, which is the maximum Parameters Such electrode Spacmgi electrode config' hardness generally prescribed f soft A uration, etc. Practice may follow convention 1n all referred upper limit of 1 ppm is prescribed since lead in- Spects Pther f addtlonfor example Elec' clusions in the deposit, while still detectable, are suffitroplatmg Engineering Handbook 3rd ciently low to have only minimal effect on the deposit. Van Nostrand RemhPld Y (1971) Below a level of about 500 ppb of Pb in solution, deposfor convemlonal 8 Platmg conslderatlonsits contain practically no lead detectable by ordinary P means. This is, therefore, a more preferred maximum A Standard? profledure was mulled for p limit. A preferred minimum is about 100 ppb since this Son PP P "mung the enumerated examples Set level is adequate to permit maximum plating rates and forth tabular form below- Platmg was on 3 Hull cell is sufficiently high so that the rate of depletion does not Panel Hull & Cleveland, Ohio 44102) require lead replenishing more frequently than the con- 30 50 disposed relative to the anode as to result in a ventional rate of gold replenishing. rent density range of from 1 to 22 mA/cm across the Load may be added in any form which is soluble in panel. In each instance, the deposition was bright yelthe solution. It is generally present as Pb or Pb(0l-l) low and otherwise of acceptable electronic characterisand/or Pb(OH) By reason of the very small amount of tics over the entirety of the panel at the conclusion of additive material, it is convenient to prepare a stock sothe example. Ineach instance, plating was carried out lution which may then be diluted as needed. It has been in a 0.6 liter solution utilizing a non-disposable anode found convenient to make additions in the form of of platinized titanium. In all instances, the solution utimeasured quantities ofa 1.000 solution prepared lized was 20 g/l of KAu(CN) TABLE Example Buffer Salt Compensating Agent Temperature No. Formula Amount g/l Formula Amount g/l pH "C KHQPO, m0 KOH 5o 10 657() 2 KHZPO, 100 KOH 7 6570 3 KH2PO, 10o 4.3 65-70 4 (NH4)2HC,,H -,O, 50 (Nrnnso, 50 4.8-5 65-70 5 NH.H. .P0, x5 4.5 65-70 a NH,H2PO, 85 Cone. NH,OH 27 ml 7 65-70 7 loco, 5n HCQ, 50 9.5 6570 a cnflcoox 5o cn coon 2 ml 65-70 9 KCN KOH 5 13 65-70 from 1.077 g/l PbO dissolved in 0.1 molar KOl-I. The examples set forth above represent but a small 3. Procedure fraction of runs made under a variety of conditions, Lead maintenance, in accordnace with the invention, with other test substrates, such as Rodar discs, upon imposes no additional restrictions on procedure. In which dependence was had for reaching conclusions fact, as indicated, somewhat greater flexibility in curand setting limits. rent density is permitted, e.g., up to about 40 mA/cm Atomic absorption and also emission spectroscopy with appropriate agitation. Such a density resulted in was utilized for determining lead content in gold deposhigh quality deposits where the cathode was itself pulits. While standard methods are unreliable for measursated, for example, over an amplitude of approximately ing lead levels of less than 100 ppm, averaging over a 2.5 cm with a periodicity of approximately 200 pulses large number of samples was considered to produce acper minute. Aside from this one change, which is opceptable results. As indicated above, solution lead levtional, apparatus and procedure may be conventional. els of less than about 400 to 500 ppb resulted in less in general, it is uneconomical to plate at very low rates than 20 ppm lead in the deposit. Lead content in the and, for this reason, it is uncommon to utilize current densities of less than about 1 mA/cm A current density of approximately 10 mA/cm may be utilizedwithdeposit was found to be substantially independent of current density at a constant concentration of lead in solution.

Increasing lead content in the solution to values above 500 ppb resulted in proportional increase in the deposit. For example, deposit lead content increased from about 10 ppm to about 30 ppm for solution lead content increase of from 500 ppb to about 1 ppm. The relationship for larger concentrations was found to be approximately linear i The linear relationship teaches that lead co-deposits readily at such low concentrations. It is reasonable to conclude that the deposition rate for lead is diffusionlimited.

Carbon content in deposits, also at a low level when plating in high quality solutions designed for electronic device use, is apparently lowered still further when baths are modified in accordance with the invention.

.Whereas extensively used, soft gold plating baths can taining lead content within the desirable level, for ex-- ample, by concurrent replenishment of both gold and lead by use of a stock solution.'A precise monitoring technique formaintenance of a bath during 'a large number of turnovers or for determining a desirable schedule for replenishment is dependent on cathode potential. It has been indicated that this potential value is a very sensitive indicator of plating morphology, for example, under the conditionsset forth, the morphology change was found to occur at between 900 millivolts and --950 millivolts. It may be expedient to direct monitor this potential and to replenish the solution accordingly. Replenishment may be manual or automatic.

What is claimed is:

1. Procedure for the electrolytic plating of soft gold comprising biasing a first electrode to be plated cathodic relative to a second electrode, both electrodes being at least partially-immersed within an aqueous solution having a pH of from 4.0 to 14.0 and containing dicyanoaurate, characterized in that said solution consists essentially of the constituents set forth together with lead contained at least in part as a dissolved compound with the lead content expressed in terms of the element being maintained in solution by dissolving during substantially the entirety of said procedure at a level of between about 20 ppb and 2 ppm based on the said solution.

. group consisting of (monobasic potassium phosphate) KH PO (dibasic potassium phosphate) K2HPO4 (tribasic potassium phosphate) K PO (dibasic ammonium citrate) (NH HC H O (ammonium phosphate) NH H PO (potassium carbonate) K CO (pota'ssium bicarbonate) KHCO (potassium cyanide) KCN, (potassium acetate) CH COOK, and (ammonium sulfate) (NI-l SO.',.

4. Procedure of claim 3 in which the buffer system includes a pH adjusting agent selected from the group consisting ofi(phosphoric acid) H PO (acetic acid) CH COOH, (potassium hydroxide) KOH, and (ammonium hydroxide) NH OH.

5. Procedure of claim 1 in which the said lead level is maintained within the said range over a period of use corresponding with gold replenishment equivalent to at least three times the amountof gold initially in the said solution.

6. Procedure of claim 5 in which the amount of gold present in the said solution is maintained between a level of from 3 grams per liter to grams per liter based on the content of dicyanoaurate in the solution.

7. Procedure of claim 1 in which the said level is maintained by addition of lead-containing material in excess of that in contact with the solution prior to addition.

maintained by dissolving lead-containing material from a reservoir in contact with the solution.

9. Procedure of claim 1 in which the said dicyanoaurate contains potassium as cation.

10. Procedure of claim 1 in which the cathode poten- 8. Procedure of claim 1 in which the said level is NI ED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,833,487 Dated Sebtember 3. 197

Inv tor) Horst Alfred Reinheimer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 40, change "10" to read -ten--.

Column 3, line 60, change ."g/ to read -g/1-.

Column 5} line 32, change "Load" to-read '--Lead--.

Signed and sealed this 17th day of December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN attesting Officer Commissioner of Patents FORM Po-mso (10-69) USCOMWDC 6037M, Q 9 U.S.'GOVIRNIINY PRINTING OFFICE II! O-Jii-BSI, 

2. Procedure of claim 1 in which the aqueous solution comprises from 15 grams per liter to 250 grams per liter of a buffer system.
 3. Procedure of claim 2 in which the said buffer system includes as pH stabilizing and conductivity providing ingredient at least one compound selected from the group consisting of (monobasic potassium phosphate) KH2PO4, (dibasic potassium phosphate) K2HPO4 (tribasic potassium phosphate) K3PO4, (dibasic ammonium citrate) (NH4)2HC6H5O7, (ammonium phosphate) NH4H2PO4, (potassium carbonate) K2CO3, (potassium bicarbonate) KHCO3, (potassium cyanide) KCN, (potassium acetate) CH3COOK, and (ammonium sulfate) (NH4)2SO4.
 4. Procedure of claim 3 in which the buffer system includes a pH adjusting agent selected from the group consisting of (phosphoric acid) H3PO4, (acetic acid) CH3COOH, (potassium hydroxide) KOH, and (ammonium hydroxide) NH4OH.
 5. Procedure of claim 1 in which the said lead level is maintained within the said range over a period of use corresponding with gold replenishment equivalent to at least three times the amount of gold initially in the said solution.
 6. Procedure of claim 5 in which the amount of gold present in the said solution is maintained between a level of from 3 grams per liter to 120 grams per liter based on the content of dicyanoaurate in the solution.
 7. Procedure of claim 1 in which the said level is maintained by addition of lead-containing material in excess of that in contact with the solution prior to addition.
 8. Procedure of claim 1 in which the said level is maintained by dissolving lead-containing material from a reservoir in contact with the solution.
 9. Procedure of claim 1 in which the said dicyanoaurate contains potassium as cation.
 10. Procedure of claim 1 in which the cathode potential is maintained at a value less negative than -930 millivolts with reference to a saturated calomel electrode during the entirety of the said procedure.
 11. Procedure of claim 1 in which the said aqueous solution contains additive material for maintaining ionic conductivity at at least 0.015 Mhos at a temperature of 25*C. 